Regarding mixing for resin infusion, isn't the fine mixing done as it infuses through all the composite fabrics?

The short answer is no. You need to mix the resin thoroughly before it gets infused.
Sure there's probably some turbulence during the infusion that could do a bit of mixing, but I wouldn't rely on that for consistent results.

Probably depends on your resin system too. The Prime 20 that we use requires fairly vigorous mixing. It's almost like mixing oil and water.. Even with a drill it takes a few minutes of mixing to get an even mix without streaks.